Changing the nickname that's shown to other people

• your centralized Google Account has a nickname associated with it
• you actually have a different nickname for every Google Groups group you join

Multiple accounts

• there are various and sundry "alternate usernames", just to make things more confusing
• no, you can't merge Google Accounts. There are ways to effectively merge some products, however:
  • GMail — go to Settings > Forwarding > Forwarding. Tell it to forward everything to your main Google Account, and tell it to "delete [this] Gmail's copy".
  • Google Calendar
    1. Settings > Calendars > (pick one at a time) > Share This Calendar > Share with Specific people > add your main account > change permissions to "Make changes to events"